1. Technical Field
The field of this invention is communication and telemetry systems, such as satellite communications systems, and, in particular, to automatic gain control circuits used with fiber optic links employed in such systems.
2. Description of the Prior Art
In communications and telemetry systems frequency and/or polarization diversity are often used to improve the reception of radio frequency (RF) signals from a mobile source (such as aircraft, vessel or automobile). This technique requires that the receiving equipment compare the relative strengths of the different received signals and combine them in a manner that results in the greatest signal-to-noise ratio.
The emergence of laser fiber-optic technology has enabled systems designers to locate the receiving equipment that does the comparing and combining great distances from the receiving antennas. Because fiber-optic links are noisy, it is very desirable to transmit high strength or high power signals. The current state-of-the-art in fiber optic equipment, however, limits the range of signal strength that can be transmitted over the fiber-optic link and forces the system designer to place variable gain elements in front of the fiber-optic transmitter to limit the transmitted signal strength. This is necessary to simultaneously keep from overdriving or burning out the fiber-optic link when the received signal strength is high and to preserve system noise temperature when the signal is weak.
With current designs, each signal channel utilizes independent automatic gain control circuits to boost the received signal strength. This gain control on the channels disturbs the relative levels of each channel at the input to the comparing and combining equipment resulting in incorrect decisions occurring at such equipment.
FIG. 1 illustrates a typical prior art two channel automatic gain control (AGC) system, generally designated 10. Left and right channels 12 and 14 are designated by the dashed lines in FIG. 1. Channel 12 consists of an automatic gain control unit 20 and a receiver 22. The input 24 to AGC unit 20 receives the left channel signal, normally an RF signal, from a receiving antenna amplifier (not shown). The output 26 of automatic gain control unit 20 is connected to the input 28 of receiver 22 via optical transducer 23 and fiber optic link 30. The output 32 of receiver 22 is connected to the input 34 of a diversity combiner 36. Right channel 14 consists of AGC unit 40 and receiver 42. The input 44 of AGC unit 40 receives the right channel signal from the antenna amplifier. The output 46 of AGC unit 40 is connected to the input 48 of receiver 42 via optical transducer 43 and optical link 50. The output 52 of receiver 42 is connected to the input 54 of diversity combiner 36. In addition, the receivers 22 and 42 also measure the signal strength of the respective received signals and transmit this data to diversity combiner unit 36 via leads 35 and 55, respectively. Transducers 23 and 43 convert the attenuated RF output signals on outputs 26 and 46 to optical signals.
On a two channel system 10 assume that the incoming or received signals on the left channel 12 and right channel 14 have power levels of +12 dBm and -32 dBm, respectively, or a signal power differential of 44 dB in signal strength. Because the right channel signal is weaker, its signal-to-noise ratio is lower than that of the left channel signal. However, using the independent AGC circuits of FIG. 1, the signals on outputs 26 and 46 of AGC units 20 and 40, respectively would each be attenuated as is known in the art (in this case amplified) to a predetermined signal strength levels of +15 dBm, for example, with a resulting differential of zero dB. The signals are each then transmitted over their respective fiber-optic links 30 and 50 to their respective receivers 22 and 42. In the diversity combiner 36, the combiner uses a weighting algorithm, as is known in the art, which combines a portion of each channel signal in proportion to its signal strength to achieve an optimal signal. To have the best signal (i.e., one with the greatest signal to noise ratio), the diversity combiner should use a greater proportion of the stronger of the two signals as originally received (i.e., the signal on left channel 12). However, because the two signals received at inputs 34 and 54 from receivers 22 and 42 have been attenuated and are now equal, the diversity combiner weighs each signal equally with the resulting combined signal at the output 56 having an increased noise content.
It would be beneficial to have an AGC circuit that maintains the relative difference in the power levels between the signals to be compared, while keeping the fiber-optic links from being overdriven.